I. Field
The following description relates generally to wireless communications, and more particularly to inter-cell power control for interference management in an OFDM system.
II. Background
Typical load control scenarios focus either on closed loop or open loop controls. There are limited views on incorporating both types of loop control. In non-orthogonal systems, methods involving both type of loop control are geared to spread spectrum time division systems and are aimed at single cell applications. In orthogonal systems, there are two main schools of thought as to uplink interference control. One camp favors a closed loop PSD control while the other favors an open loop PSD control. Each method has its advantages and disadvantages.
Typically, closed loop power control methodologies are very fast and there is a perception that there is little need for an open loop control method. However, there are concerns with accuracy of closed loop control and that without a proper starting point, the closed loop approach may not be fast enough.
In a representative open loop power control method, the end node uses the measured total received power along with typical values of certain base station parameters to get a rough estimate of the transmission loss between the end node and the base station. Based on these measurements, the forward link transmission loss is estimated and used to determine the proper open loop power control setting for the end node transmitter. The end node's transmit power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level. All end nodes within a cell use the same process, and ideally their signal will arrive with equal power at the base station.
The base station parameters typically contain correction factor(s) to be used by the end nodes in its open loop power estimate on an ongoing as well as for the initial transmission on an access channel. Conventional algorithms exist for estimating the end node's desired transmit power for the first access probe on the access channel. It should be noted that the value of the open loop power control constant depends on many dynamically varying parameters (including for example cell layout, network load, location of end node within the cell.) None of these dynamically changing variables is known, a priori, hence, the first probe power level will likely be in error. The error may result in a far too high of a power level than necessary to establish communications when the mobile station is close to the base station. When the transmit power level is too high, unnecessary interference to the remaining mobile stations is created, reducing the capacity of the system. On the other hand, if the mobile station is far away, it may transmit the initial access probe at too low a power level, resulting in additional probes being sent. In addition to increasing call setup time, additional probes will result in more reverse link interference.
Urban Canyon areas are also in need of improved control, where the geometry of the cell coverage may impose dynamic and unreliable load indications on the end node's traveling in the area. Turning a corner and blasting the heightened cell phone transmit power into a neighboring cell needs a better control mechanism as controlling by a single serving cell is not adequate.
Therefore, in order to maximize the effectiveness of user experience, it may be appreciated from the foregoing discussion that the problems with interference from other cells and weak signals from the serving cell at a cell's edge should be considered in more detail for desired control methodology than the current state of the art provides.